KABUL, Afghanistan (CNN) -- Four NATO service members -- three Britons and one American -- were killed Thursday in separate attacks in southern Afghanistan, according to NATO and Britain's Defence Ministry said.

The three British soldiers died in an explosion while on patrol near Sangin in Helmand Province, the ministry said. NATO said it was a roadside bomb, or improvised explosive device, that caused the blast.

"We will all feel the loss of these brave soldiers, but it is their family, friends and loved ones, as well as the men and women who served alongside them, who feel the greatest pain and we offer them our deepest and heartfelt condolences, thoughts and prayers," Task Force Helmand spokesman Lt. Col. Nick Richardson said.

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The U.S. service member was killed in a direct fire attack, according to NATO's International Security Assistance Forces. There were no further details about the attack.

ISAF spokesman Brig. Gen. Eric Tremblay offered his condolences to the families of all the service members.

"While we will always honor their sacrifices, we will stay focused on helping ensure a better future for the people of Afghanistan, free of Taliban harassment and repression."

Explosions of roadside bombs -- also called improvised explosive devices, or IEDs -- have increased by 114 percent in Afghanistan this year compared to the same period last year, the British Ministry of Defence said late last month.

Of troops killed in action, 73 percent of them were killed by IEDs, the ministry said.

The U.S. military has called IEDs the weapon of choice for insurgents in Afghanistan and the biggest threat facing coalition troops there.